Energy conversion in the functional membrane of photosynthesis. Analysis by light pulse and electric pulse methods: The central role of the electric field

Alkalization of the Chloroplast Stroma Caused by Light-Dependent Proton Flux Into the Thylakoid Space

In nature, plants are challenged by constantly changing light conditions. To reveal the molecular mechanisms behind acclimation to sometimes drastic and frequent changes in light intensity, we grew Arabidopsis thaliana under fluctuating light conditions, in which the low light periods were repeatedly interrupted with high light peaks. Such conditions had only marginal effect on photosystem II but induced damage to photosystem I (PSI), the damage being most severe during the early developmental stages. We showed that PROTON GRADIENT REGULATION5 (PGR5)–dependent regulation of electron transfer and proton motive force is crucial for protection of PSI against photodamage, which occurred particularly during the high light phases of fluctuating light cycles. Contrary to PGR5, the NAD(P)H dehydrogenase complex, which mediates cyclic electron flow around PSI, did not contribute to acclimation of the photosynthetic apparatus, particularly PSI, to rapidly changing light intensities. Likewise, the Arabidopsis pgr5 mutant exhibited a significantly higher mortality rate compared with the wild type under outdoor field conditions. This shows not only that regulation of PSI under natural growth conditions is crucial but also the importance of PGR5 in PSI protection.

PROTON GRADIENT REGULATION5 Is Essential for Proper Acclimation of Arabidopsis Photosystem I to Naturally and Artificially Fluctuating Light Conditions

Coupling of quanta, electrons, fields, ions and phosphorylation in the functional membrane of photosynthesis. Results by pulse spectroscopic methods

Proton motive force (pmf), established across the thylakoid membrane by photosynthetic electron transfer, functions both to drive the synthesis of ATP and initiate processes that down-regulate photosynthesis. At the same time, excessively low lumen pH can lead to the destruction of some lumenal components and sensitization of the photosynthetic apparatus to photoinhibition. Therefore, in order to understand the energy budget of photosynthesis, its regulation and responses to environmental stresses, it is essential to know the magnitude of pmf, its distribution between ΔpH and the electric field (Δϕ) as well as the relationships between these parameters and ΔGATP, and down-regulatory and inhibitory processes. We review past estimates of lumen pH and propose a model that can explain much of the divergent data in the literature. In this model, in intact plants under permissive conditions, photosynthesis is regulated so that lumen pH remains mod erate (between 5.8 and 6.5), where it modulates the activity of the violaxanthin deepoxidase, does not significantly restrict the turnover of the cytochrome b6f complex, and does not destabilize the oxygen evolving complex. Only under stressed conditions, where light input exceeds the capacity of both photosynthesis and down-regulatory processes, does lumen pH decrease below 5, possibly contributing to photoinhibition. A value of n = 4 for the stoichiometry of protons pumped through the ATP synthase per ATP synthesized, and a minor contribution of Δϕ to pmf, will allow moderate lumen pH to sustain the observed levels of ΔGATP.

U. Siggel

Papers

pH changes in the inner phase of the thylakoids during photosynthesis